I have a little doubt of this arrangement (please the attachment) . In this circuit C1 capacitor is used as a “frequency defendant resister” and the R1 resister is used as a “bleeder resister”, I suppose! So , how do I calculate the resistance between A and B.

Can I do it like this?

1/R = 1/Rc + 1/R1 {Rc=1/2∏FC} ; R1= 1M

 

Hello,

Take a look at this page of the eBook:
http://www.allaboutcircuits.com/vol_2/chpt_4/4.html

Bertus

PS keep in mind that we do not want circuits directly connected to the mains.
For safety always use a transformer.